1. Field of the Invention
The present invention relates to an illumination optical apparatus for use in display optical apparatus employing a display panel.
2. Description of the Prior Art
As one of conventional means for displaying an image, projection-type display optical apparatuses are known. Such display optical apparatuses require the use of an illumination optical apparatus to illuminate efficiently and evenly an optical image formed on a reflection-type liquid crystal display panel or the like. FIG. 11 schematically shows the sectional configuration of an illumination optical apparatus employed in a conventional display optical apparatus.
In this figure, the light emitted from a light source 1 is reflected by a reflector 2 so as to be directed, as a substantially parallel beam of non-polarized light, to the illumination optical apparatus OP1. The illumination optical apparatus OP1 is composed of, in the order in which the light beam passes through, a UV/IR (ultraviolet and infrared) cut filter 7, a birefringent diffraction grating 3, a first lens array 4, half-wave plates 5, and a second lens array 6.
The birefringent diffraction grating 3 has a blazed diffraction grating 101 (i.e. a diffraction grating having blaze-shaped grooves) formed on a substrate 100 made of glass or the like, and has a birefringent optical material 102 sealed in a portion (hatched) thereof between the blazed side of the substrate 100 and a glass plate 103. The birefringent optical material 102 exhibits different refractive indices for light polarized in different directions (i.e. light having different polarization planes); specifically, in the example shown here, the birefringent optical material 102 exhibits different refractive indices for rays L1 having a polarization plane parallel to the plane of the figure as indicated by arrows and for rays L2 having a polarization plane perpendicular to the plane of the figure as indicated by center-dotted circles. On the other hand, the diffraction grating 101 is so shaped as to deflect light that has been traveling straight.
Here, the refractive index for the rays L1 having a polarization plane parallel to the plane of the figure is made equal to the refractive index of the material of the substrate so that the rays L1 having a polarization plane parallel to the plane of the figure will travel as if there were no diffraction grating 101, as indicated by solid lines, and that the rays L2 having a polarization plane perpendicular to the plane of the figure will travel under the influence of the diffraction gating 101 so as to be deflected thereby, as indicated by broken lines.
The first lens array 4, disposed next to the birefringent diffraction grating 3, divides spatially the rays incident thereon and focuses them on the second lens array 6. Here, whereas the rays L1 having a polarization plane parallel to the plane of the figure is allowed to travel straight before being focused, the rays L2 having a polarization plane perpendicular to the plane of the figure is deflected before being focused. Accordingly, the rays L1 having a polarization plane parallel to the plane of the figure and the rays L2 having a polarization plane perpendicular to the plane of the figure are focused in spatially different positions. Thus, by disposing the half-wave plates 5 near the second lens array 6 on the light-source side thereof in such a way as to cover only the positions where the rays L1 or L2 having either of the two polarization planes described above are focused, it is possible to make uniform the polarization plane of all of the rays L1 and L2.
As a result, the illumination optical apparatus OP1 emits, as illumination light, light that is wholly polarized parallel to the plane of the figure. A birefringent optical material is obtained, for example, by orienting a liquid crystal material in a predetermined direction. This may be done by the use of a liquid crystal material that is known to harden when subjected to ultraviolet or other radiation; in that case, the liquid crystal material is subjected to ultraviolet or other radiation after the orientation mentioned above.
FIG. 12 is an exploded perspective view schematically showing the relationship among the birefringent diffraction grating and the first and second lens arrays of the conventional illumination optical apparatus described above. In this figure, only part of the lens cells constituting the lens arrays are shown as their representatives. In this figure, the rays L0 coming from the light source 1 and the reflector 2, which are disposed on the lower left side of the figure but not shown here, are separated, by the polarization plane separation action of the blazes 3a of the birefringent diffraction grating 3, into rays 1 having a predetermined polarization plane, indicated by solid lines, and rays 2 having a polarization plane perpendicular thereto, indicated by broken lines.
These rays pass through individual lens cells A, B, C, and D arranged in a grid-like formation in the first lens array 4 and then form, on each of individual lens cells Aa, Ba, Ca, and Da arranged in a similar grid-like formation in the second lens array 6, a pair of a light-source image having the predetermined polarization plane and a light-source image having the polarization plane perpendicular thereto. In each pair, the two light-source images lie apart from each other in an exact row along the direction in which the birefringent diffraction grating 3 separates the rays. Moreover, as indicated by solid-line and broken-line ellipses (circles if seen from the front side of the lens array), these light-source images are projected on the individual lens cells of the second lens array 6 in such a way as to have an appreciable size. It is to be noted that, in the example under discussion, with respect to each optical element as seen from the light source side, a coordinate system is assumed to have a y axis pointing upward and an x axis pointing rightward.
However, in this conventional illumination optical apparatus, as described above, the separated light-source images are projected on the individual lens cells of the second lens array 6 in such a way as to have an appreciable size each, and therefore the projected light-source images overlap each other. To make uniform the polarization plane of all of these light-source images, it is necessary to dispose half-wave plates, like the half-wave plates 5 shown in FIG. 11, in such a way as to cover only the light-source images having either of the two polarization planes described above to convert their polarization plane. However, in the regions where the light-source images overlap each other, it is impossible to make the polarization plane uniform, and therefore such regions are useless. That is, the rays that pass through such regions, when they eventually reach the display panel, either have a wrong polarization plane or strike outside the effective area of the display panel, and thus do not serve as illumination light. This degrades illumination efficiency.
One way to achieve higher illumination efficiency in such a conventional illumination optical apparatus is to use a lens array having irregular apertures. For example, Japanese Laid-Open Patent Application No. H5-346557 proposes achieving higher illumination efficiency by exploiting the fact that the above-mentioned light-source images have different sizes near the center and near the edge of the second lens array, specifically by designing the second lens array to have accordingly irregular apertures at uneven intervals.
However, this method is by nature unsuitable for polarization plane conversion because the light-source images projected on the second lens array do not form an exact row and therefore it is difficult to attach the half-wave plates in the positions of the light-source images whose polarization plane needs to be converted. By contrast, in the conventional illumination optical apparatus as originally described above or in the embodiment of the present invention described later, where the light-source images form an exact row, it is possible to use strip-shaped half-wave plates and thus it is easy to attach the half-wave plates.